Multiple transmit and receive antennas have been proposed to provide both increased robustness and capacity in next generation Wireless Local Area Network (WLAN) systems. The increased robustness can be achieved through techniques that exploit the spatial diversity and additional gain introduced in a system with multiple antennas. The increased capacity can be achieved in multipath fading environments with bandwidth efficient Multiple Input Multiple Output (MIMO) techniques. A multiple antenna communication system increases the data rate in a given channel bandwidth by transmitting separate data streams on multiple transmit antennas. Each receiver receives a combination of these data streams on multiple receive antennas.
In order to properly receive the different data streams, receivers in a multiple antenna communication system must acquire the channel matrix through training. This is generally achieved by using a specific training symbol, or preamble, to perform synchronization and channel estimation. The preamble helps the receiver (i) estimate the power of the received signal to set an automatic gain control (AGC) function; (ii) acquire the timing offset to perform optimal placement of a Fast Fourier Transform (FFT) window; (iii) estimate the frequency offset between the transmitter and receiver, and correct for the frequency offset prior to FFT demodulation; and (iv) estimate the channel transfer function to help demap the Quadrature Amplitude Modulation (QAM) symbols after the FFT has been performed.
In addition, a number of pilot tones are embedded in the OFDM data symbols to estimate the phase noise and residual frequency offset. Phase noise at the local oscillators of the transmitter and receiver creates a common phase error (CPE) at the FFT output that generally needs to be corrected for every OFDM symbol. Residual frequency offset at the input of the FFT also creates CPE. In general, the accuracy of the CPE estimation increases with the number of pilots, thereby reducing the packet error rate, and increasing the reliability of the transmission.
Generally, MIMO systems transmit the same pilot tones and polarization sequence on all the antennas. The pilots are a determined signal. Thus, there are certain beam patterns of the pilots. In a frequency selective channel, different pilot tones will experience different channels. Thus, each pilot tone has a different beam pattern. Therefore, some pilots will be enhanced by the channel while other pilots will be cancelled. It has been observed that the beam forming is more sever in the case of “flat fading” channels. In this case, all the pilots experience the same channel fading and can all be cancelled out. Thus, although the channel conditions allow the receiver to receive the data correctly, the receiver may not be able to process the data because the pilots are all faded.
Generally, MIMO systems transmit the same pilot tones and polarization sequence on all the antennas. The pilots are a deterministic signal. Thus, if the channel from multiple transmit antennas to a given receive antenna is highly correlated, the pilots will create certain beam pattern in the far field. Therefore, as a function of the azimuth angle in the two dimensional plane, some pilots will be enhanced by the channel while other pilots will be degraded. It has been observed that the beam forming is most severe in the case of “flat fading” channels whereby the channel does not change as a function of frequency. In this case, all the pilots experience the same channel fading and can cancel out as specific azimuth angles. Thus, although the channel conditions allow the receiver to receive the data correctly, the receiver may not be able to process the data because of catastrophic fading on the pilots.
A need therefore exists for methods and apparatus for communicating orthogonal pilot tones in a multiple antenna communication system, such that the pilot tones will not cancel one another in the channel.